Wafer processing method

ABSTRACT

A wafer is divided into a plurality of individual devices along a plurality of crossing division lines formed on the front side of the wafer. The wafer has a substrate, a functional layer formed on the front side of the substrate, and an SiO 2  film formed on the front side of the functional layer. The individual devices are formed from the functional layer and partitioned by the division lines. The functional layer is removed by applying a laser beam to the wafer along each division line to thereby remove the functional layer along each division line. The laser beam has an absorption wavelength to the SiO 2  film with high absorptivity due to the stretching vibration of an O—H bond or a C—H bond remaining in the SiO 2  film. The wafer is then divided into the individual devices.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a wafer processing method for dividinga wafer into a plurality of individual devices along a plurality ofcrossing division lines formed on the front side of the wafer, the waferbeing composed of a substrate and a functional layer formed on the frontside of the substrate, the individual devices being formed from thefunctional layer and partitioned by the division lines.

2. Description of the Related Art

As well known in the art, in a semiconductor device fabrication process,a functional layer composed of an insulating film and a functional filmis formed on the front side of a substrate such as a silicon substrate,and a plurality of devices such as ICs and LSIs are formed like a matrixfrom this functional layer, thus obtaining a semiconductor wafer havingthe plural devices. The plural devices are partitioned by a plurality ofdivision lines formed on the front side of the semiconductor wafer. Thesemiconductor wafer is divided along these division lines to obtain theindividual devices.

In recent years, a semiconductor wafer intended to improve theprocessing performance of semiconductor chips (devices) such as ICs andLSIs has been put into practical use. This semiconductor wafer iscomposed of a substrate such as a silicon substrate and a functionallayer formed on the front side of the substrate, wherein the functionallayer is composed of a low-permittivity insulator film (low-k film) anda functional film formed on the low-k film, the functional film forminga plurality of circuits. Thus, the semiconductor devices are formed fromthe functional layer. The low-k film is formed from an inorganic film ofSiOF, BSG (SiOB), etc. or an organic film such as a polymer film ofpolyimide, parylene, etc.

Division of such a semiconductor wafer along the division lines isusually performed by using a cutting apparatus called a dicing saw. Thiscutting apparatus includes a chuck table for holding the semiconductorwafer as a workpiece, cutting means for cutting the semiconductor waferheld on the chuck table, and moving means for relatively moving thechuck table and the cutting means. The cutting means includes a rotatingspindle adapted to be rotated at high speeds and a cutting blade mountedon the rotating spindle. The cutting blade is composed of a disk-shapedbase and an annular cutting edge mounted on one side surface of the basealong the outer circumference thereof. The annular cutting edge is anelectroformed diamond blade formed by bonding diamond abrasive grainshaving a grain size of about 3 μm, for example.

However, it is difficult to cut the low-k film mentioned above by usingthe cutting blade. That is, the low-k film is very brittle like mica.Accordingly, when the semiconductor wafer having the low-k film is cutalong the division lines by using the cutting blade, there arises aproblem such that the low-k film may be separated and this separationmay reach the devices to cause fatal damage to the devices.

To solve this problem, Japanese Patent Laid-Open No. 2009-21476discloses a wafer dividing method including the steps of applying alaser beam along both sides of each division line on a semiconductorwafer to form two laser processed grooves along each division line,thereby dividing a stacked layer and next positioning a cutting bladebetween the outer side walls of the two laser processed grooves alongeach division line to relatively move the cutting blade and thesemiconductor wafer, thereby cutting the semiconductor wafer along eachdivision line.

SUMMARY OF THE INVENTION

However, the functional layer including the low-k film is covered withan SiO₂ film. Accordingly, when a laser beam is applied to thefunctional layer, the laser beam passes through the SiO₂ film to reachthe inside of the functional layer. As a result, heat is generated bythe application of the laser beam to the low-k film and the substrateand this heat is confined in the functional layer by the SiO₂ film, sothat there is a possibility of thermal expansion of the functional layerto cause the separation of the functional layer in the area where thecircuits are formed and the density is low.

It is therefore an object of the present invention to provide a waferprocessing method which can divide a wafer into the individual devicesalong the division lines without the separation of a functional layer ineach device, wherein the wafer includes a substrate and the functionallayer formed on the front side of the substrate and the devices areformed from this functional layer.

In accordance with an aspect of the present invention, there is provideda wafer processing method for dividing a wafer into a plurality ofindividual devices along a plurality of crossing division lines formedon the front side of the wafer, the wafer including a substrate, afunctional layer formed on the front side of the substrate, and an SiO₂film formed on the front side of the functional layer, the individualdevices being formed from the functional layer and partitioned by thedivision lines, the wafer processing method including a functional layerremoving step of applying a laser beam to the wafer along each divisionline to thereby remove the functional layer along each division line,the laser beam having an absorption wavelength to the SiO₂ film withhigh absorptivity due to the stretching vibration of an O—H bond or aC—H bond remaining in the SiO₂ film; and a wafer dividing step ofprocessing the substrate along each division line where the functionallayer has been removed by performing the functional layer removing step,thereby dividing the wafer into the individual devices.

Preferably, the wavelength of the laser beam to be applied in thefunctional layer removing step is set to 2.6 to 3.5 μm Preferably, thewafer dividing step includes the step of applying a laser beam having anabsorption wavelength to the substrate along each division line wherethe functional layer has been removed, thereby ablating the substrate toform a division groove on the substrate along each division line.

As another preferred configuration, the wafer dividing step includes thestep of applying a laser beam having a transmission wavelength to thesubstrate along each division line where the functional layer has beenremoved in the condition where a focal point of the laser beam is setinside the substrate, thereby forming a modified layer inside thesubstrate along each division line.

As still another preferred configuration, the wafer dividing stepincludes the step of aligning a cutting blade with each division linewhere the functional layer has been removed, thereby cutting thesubstrate along each division line.

In the functional layer removing step of the wafer processing methodaccording to the present invention, the laser beam having an absorptionwavelength to the SiO₂ film with high absorptivity due to the stretchingvibration of the O—H bond or the C—H bond remaining in the SiO₂ film isapplied to the wafer along each division line. Accordingly, when thislaser beam is applied to the SiO₂ film formed on the front side of thefunctional layer, the laser beam is not passed through the SiO₂ film,but absorbed by the SiO₂ film. Accordingly, the SiO₂ film is ablatedinstantaneously and does not confine the heat inside the functionallayer, thereby eliminating the possibility of separation of thefunctional layer in the area where the circuits are formed and thedensity is low.

The above and other objects, features and advantages of the presentinvention and the manner of realizing them will become more apparent,and the invention itself will best be understood from a study of thefollowing description and appended claims with reference to the attacheddrawings showing some preferred embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a perspective view of a semiconductor wafer to be divided bya wafer processing method according to the present invention;

FIG. 1B is an enlarged sectional view of an essential part of thesemiconductor wafer shown in FIG. 1A;

FIG. 2 is a perspective view showing a condition where the semiconductorwafer which a wafer supporting step is performed is attached to a dicingtape supported to an annular frame;

FIG. 3 is a perspective view of an essential part of a laser processingapparatus for performing a functional layer removing step;

FIGS. 4A to 4C are views for illustrating the functional layer removingstep;

FIG. 5 is a perspective view of an essential part of a laser processingapparatus for performing a first preferred embodiment of a waferdividing step;

FIGS. 6A to 6C are views for illustrating a division groove forming stepas the first preferred embodiment of the wafer dividing step;

FIGS. 7A to 7C are views for illustrating a modified layer forming stepas a second preferred embodiment of the wafer dividing step;

FIG. 8 is a perspective view of an essential part of a cutting apparatusfor performing a third preferred embodiment of the wafer dividing step;and

FIGS. 9A to 9D are views for illustrating a cut groove forming step asthe third preferred embodiment of the wafer dividing step.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The wafer processing method according to the present invention will nowbe described in more detail with reference to the attached drawings.FIG. 1A is a perspective view of a semiconductor wafer 2 to be dividedinto individual devices by the wafer processing method according to thepresent invention, and FIG. 1B is an enlarged sectional view of anessential part of the semiconductor wafer 2 shown in FIG. 1A. As shownin FIGS. 1A and 1B, the semiconductor wafer 2 is composed of a substrate20 such as a silicon substrate and a functional layer 21 formed on thefront side 20 a of the substrate 20. For example, the substrate 20 has athickness of 140 μm. The functional layer 21 is composed of aninsulating film and a functional film formed on the insulating film, thefunctional film forming a plurality of circuits. A plurality of devices22 such as ICs and LSIs are formed like a matrix from the functionallayer 21. These devices 22 are partitioned by a plurality of crossingdivision lines 23 formed on the functional layer 21. In this preferredembodiment, the insulating film constituting the functional layer 21 isprovided by an SiO₂ film or a low-permittivity insulator film (low-kfilm). Examples of the low-k film include an inorganic film of SiOF, BSG(SiOB), etc. and an organic film such as a polymer film of polyimide,parylene, etc. For example, the thickness of the insulating film is setto 10 μm. Further, an SiO₂ film 211 is formed on the front side of thefunctional layer 21. In forming the SiO₂ film 211, an O—H bond or a C—Hbond in a required raw material is precipitated, so that the O—H bond orthe C—H bond remains in the SiO₂ film 211.

The wafer processing method for dividing the semiconductor wafer 2 alongthe division lines 23 will now be described. First, a wafer supportingstep is performed in such a manner that a back side 20 b of thesubstrate 20 constituting the semiconductor wafer 2 is attached to adicing tape supported to an annular frame. More specifically, as shownin FIG. 2, a dicing tape 30 is supported at its peripheral portion to anannular frame 3 so as to close the inside opening of the annular frame3. The back side 20 b of the substrate 20 constituting the semiconductorwafer 2 is attached to the front side (upper surface) of the dicing tape30 supported to the annular frame 3. Accordingly, the semiconductorwafer 2 is supported through the dicing tape 30 to the annular frame 3in the condition where the SiO₂ film 211 formed on the front side of thefunctional layer 21 is oriented upward.

After performing the wafer supporting step mentioned above, a functionallayer removing step is performed in such a manner that a laser beamhaving an absorption wavelength to the SiO₂ film 211 with highabsorptivity due to the stretching vibration of the O—H bond or the C—Hbond remaining in the SiO₂ film 211 is applied along each division line23 of the semiconductor wafer 2 to remove the functional layer 21 alongeach division line 23. This functional layer removing step is performedby using a laser processing apparatus 4 shown in FIG. 3. As shown inFIG. 3, the laser processing apparatus 4 includes a chuck table 41 forholding a workpiece, laser beam applying means 42 for applying a laserbeam to the workpiece held on the chuck table 41, and imaging means 43for imaging the workpiece held on the chuck table 41. The chuck table 41has an upper surface as a holding surface for holding the workpiecethereon under suction. The chuck table 41 is movable both in the feedingdirection shown by an arrow X in FIG. 3 by feeding means (not shown) andin the indexing direction shown by an arrow Y in FIG. 3 by indexingmeans (not shown).

The laser beam applying means 42 includes a cylindrical casing 421extending in a substantially horizontal direction. Although not shown,the casing 421 contains pulsed laser beam oscillating means including apulsed laser oscillator and repetition frequently setting means. Thelaser beam applying means 42 further includes focusing means 422 mountedon the front end of the casing 421 for focusing a pulsed laser beamoscillated by the pulsed laser beam oscillating means. The laser beamapplying means 42 further includes focal position adjusting means (notshown) for adjusting the focal position of the pulsed laser beam to befocused by the focusing means 422.

The imaging means 43 is mounted on a front end portion of the casing 421constituting the laser beam applying means 42 and includes illuminatingmeans for illuminating the workpiece, an optical system for capturing anarea illuminated by the illuminating means, and an imaging device (CCD)for imaging the area captured by the optical system. An image signaloutput from the imaging means 43 is transmitted to control means (notshown).

There will now be described with reference to FIG. 3 and FIGS. 4A to 4Cthe functional layer removing step of applying a laser beam to thesemiconductor wafer 2 along each division line 23 by using the laserprocessing apparatus 4 mentioned above to thereby remove the functionallayer 21 along each division line 23, the laser beam having anabsorption wavelength to the SiO₂ film 211 with high absorptivity due tothe stretching vibration of the O—H bond or the C—H bond remaining inthe SiO₂ film 211. First, the semiconductor wafer 2 is placed on thechuck table 41 of the laser processing apparatus 4 in the conditionwhere the dicing tape 30 attached to the semiconductor wafer 2 is incontact with the chuck table 41 as shown in FIG. 3. Thereafter, suctionmeans (not shown) is operated to hold the semiconductor wafer 2 throughthe dicing tape 30 on the chuck table 41 under suction (wafer holdingstep). Accordingly, the SiO₂ film 211 formed on the front side of thefunctional layer 21 of the semiconductor wafer 2 held on the chuck table41 is oriented upward. Although the annular frame 3 supporting thedicing tape 30 is not shown in FIG. 3, the annular frame 3 is held bysuitable frame holding means provided on the chuck table 41. Thereafter,the chuck table 41 holding the semiconductor wafer 2 is moved to aposition directly below the imaging means 43 by operating the feedingmeans (not shown).

In the condition where the chuck table 41 is positioned directly belowthe imaging means 43, an alignment operation is performed by the imagingmeans 43 and the control means (not shown) to detect a subject area ofthe semiconductor wafer 2 to be laser-processed. More specifically, theimaging means 43 and the control means perform image processing such aspattern matching for making the alignment of the division lines 23 in afirst direction on the functional layer 21 of the semiconductor wafer 2and the focusing means 422 of the laser beam applying means 42 forapplying the laser beam to the wafer 2 along the division lines 23, thusperforming the alignment of a laser beam applying position (alignmentstep). Similarly, the alignment of a laser beam applying position isperformed for the other division lines 23 extending in a seconddirection perpendicular to the first direction on the functional layer21.

After performing the alignment step mentioned above, the chuck table 41is moved to a laser beam applying area where the focusing means 422 ofthe laser beam applying means 42 is located as shown in FIG. 4A, therebypositioning one end (left end as viewed in FIG. 4A) of a predeterminedone of the division lines 23 directly below the focusing means 422.Further, a focal point P of the pulsed laser beam to be applied from thefocusing means 422 is set near the upper surface of the SiO₂ film 211 inthe predetermined division line 23. Thereafter, the pulsed laser beamhaving an absorption wavelength to the SiO₂ film 211 with highabsorptivity due to the stretching vibration of the O—H bond or the C—Hbond remaining in the SiO₂ film 211 is applied from the focusing means422 to the wafer 2, and the chuck table 41 is moved in the directionshown by an arrow X1 in FIG. 4A at a predetermined feed speed. When theother end (right end as viewed in FIG. 4B) of the predetermined divisionline 23 reaches the position directly below the focusing means 422 asshown in FIG. 4B, the application of the pulsed laser beam is stoppedand the movement of the chuck table 41 is also stopped. As a result, thefunctional layer 21 is removed along the predetermined division line 23of the semiconductor wafer 2 to form a laser processed groove 24 asshown in FIG. 4C. This functional layer removing step is similarlyperformed along all of the division lines 23 formed on the semiconductorwafer 2.

In the functional layer removing step mentioned above, the laser beamapplying means 42 applies the pulsed laser beam having an absorptionwavelength to the SiO₂ film 211 with high absorptivity due to thestretching vibration of the O—H bond or the C—H bond remaining in theSiO₂ film 211. Band gaps of the O—H bond and the C—H bond have peaks ina region of 2.6 to 3.5 μm. Therefore, as the pulsed laser beam having anabsorption wavelength to the SiO₂ film 211 with high absorptivity due tothe stretching vibration of the O—H bond or the C—H bond remaining inthe SiO₂ film 211, a pulsed laser beam having a wavelength of 2.6 to 3.5μm is applied. As a result, when the pulsed laser beam is applied to theSiO₂ film 211 formed on the front side of the functional layer 21, thepulsed laser beam is not passed through the SiO₂ film 211, but absorbedby the SiO₂ film 211. Accordingly, the SiO₂ film 211 is ablatedinstantaneously and does not confine the heat inside the functionallayer 21, thereby eliminating the possibility of separation of thefunctional layer 21 in the area where the circuits are formed and thedensity is low.

For example, the functional layer removing step mentioned above isperformed under the following processing conditions.

Light source: Er: YAG laser

Wavelength of the laser beam: 2.7 μm

Repetition frequency: 50 kHz

Average power: 0.5 W

Focused spot diameter: φ50 μm

Work feed speed: 200 mm/second

After performing the functional layer removing step mentioned above, awafer dividing step is performed in such a manner that the substrate 20is processed along each division line 23 where the functional layer 21has been removed by performing the functional layer removing step,thereby dividing the semiconductor wafer 2 into the individual devices22. A first preferred embodiment of this wafer dividing step will now bedescribed with reference to FIG. 5 and FIGS. 6A to 6C.

The first preferred embodiment of the wafer processing step is the stepof applying a laser beam having an absorption wavelength to thesubstrate 20 along each division line 23 where the functional layer 21has been removed, thereby ablating the substrate 20 to form a divisiongroove on the substrate 20 along each division line 23 (division grooveforming step). This division groove forming step may be performed byusing a laser processing apparatus similar to the laser processingapparatus 4 shown in FIG. 3. Such a similar laser processing apparatusis shown in FIG. 5 and the same reference numerals as those shown inFIG. 3 are used in FIG. 5 for convenience of illustration. In performingthe division groove forming step, the semiconductor wafer 2 processed bythe functional layer removing step is placed on the chuck table 41 inthe condition where the dicing tape 30 attached to the semiconductorwafer 2 is in contact with the chuck table 41 as shown in FIG. 5.Thereafter, suction means (not shown) is operated to hold thesemiconductor wafer 2 through the dicing tape 30 on the chuck table 41under suction (wafer holding step). Accordingly, the SiO₂ film 211formed on the front side of the functional layer 21 of the semiconductorwafer 2 held on the chuck table 41 is oriented upward. Although theannular frame 3 supporting the dicing tape 30 is not shown in FIG. 5,the annular frame 3 is held by suitable frame holding means provided onthe chuck table 41. Thereafter, the chuck table 41 holding thesemiconductor wafer 2 is moved to a position directly below the imagingmeans 43 by operating the feeding means (not shown). Thereafter, analignment step is performed in the same manner as that mentioned above.

Thereafter, the chuck table 41 is moved to a laser beam applying areawhere the focusing means 422 of the laser beam applying means 42 islocated as shown in FIG. 6A, thereby positioning a predetermined one ofthe division lines 23 directly below the focusing means 422, wherein thelateral center position of the laser processed groove 24 formed alongthe predetermined division line 23 is set as a laser beam applyingposition where the laser beam is applied from the focusing means 422. Atthis time, one end (left end as viewed in FIG. 6A) of the predetermineddivision line 23 is positioned directly below the focusing means 422.Further, a focal point P of the pulsed laser beam to be applied from thefocusing means 422 is set near the front side (upper surface) of thesubstrate 20 in the predetermined division line 23 where the functionallayer 21 has been removed. Thereafter, the pulsed laser beam is appliedfrom the focusing means 422 to the wafer 2, and the chuck table 41 ismoved in the direction shown by an arrow X1 in FIG. 6A at apredetermined feed speed. In this division groove forming step, thewavelength of the pulsed laser beam is set to an absorption wavelengthto the substrate 20. When the other end (right end as viewed in FIG. 6B)of the predetermined division line 23 reaches the position directlybelow the focusing means 422 as shown in FIG. 6B, the application of thepulsed laser beam is stopped and the movement of the chuck table 41 isalso stopped.

By performing the division groove forming step, a division groove 25having a predetermined depth is formed in the substrate 20 along thelaser processed groove 24 formed along the predetermined division line23 as shown in FIG. 6C. The division groove forming step mentioned aboveis similarly performed along all of the division lines 23 of thesemiconductor wafer 2 processed by the functional layer removing step.

For example, the division groove forming step mentioned above isperformed under the following processing conditions.

Light source: YAG laser

Wavelength: 355 nm (third harmonic generation

of YAG laser)

Repetition frequency: 100 kHz

Average power: 1.2 W

Focused spot diameter: φ10 μm

Work feed speed: 200 mm/second

After performing the division groove forming step as mentioned above, anexternal force is applied to the semiconductor wafer 2 to thereby dividethe semiconductor wafer 2 along each division line 23 where the divisiongroove 25 is formed. That is, since the division groove 25 as a divisionstart point is formed along each division line 23 on the semiconductorwafer 2, the semiconductor wafer 2 can be easily divided along eachdivision line 23 by applying the external force. As a modification, thedivision groove 25 may be formed over the entire thickness of thesubstrate 20 of the semiconductor wafer 2 along each division line 23 inthe division groove forming step, thereby cutting the substrate 20 alongeach division line 23.

A second preferred embodiment of the wafer dividing step will now bedescribed with reference to FIGS. 7A to 7C. The second preferredembodiment of the wafer dividing step is the step of applying a laserbeam having a transmission wavelength to the substrate 20 along eachdivision line 23 where the functional layer 21 has been removed in thecondition where a focal point of the laser beam is set inside thesubstrate 20, thereby forming a modified layer inside the substrate 20along each division line 23 (modified layer forming step). This modifiedlayer forming step may be performed by using a laser processingapparatus similar to the laser processing apparatus 4 shown in FIG. 3 orFIG. 5. In performing the modified layer forming step, the semiconductorwafer 2 processed by the functional layer removing step is placed on thechuck table 41 in the condition where the dicing tape 30 attached to thesemiconductor wafer 2 is in contact with the chuck table 41 as in thedivision groove forming step shown FIG. 5. Thereafter, suction means(not shown) is operated to hold the semiconductor wafer 2 through thedicing tape 30 on the chuck table 41 under suction (wafer holding step).Accordingly, the SiO₂ film 211 formed on the front side of thefunctional layer 21 of the semiconductor wafer 2 held on the chuck table41 is oriented upward. Thereafter, an alignment step is performed in thesame manner as that mentioned above.

Thereafter, the chuck table 41 is moved to a laser beam applying areawhere the focusing means 422 of the laser beam applying means 42 islocated as shown in FIG. 7A, thereby positioning a predetermined one ofthe division lines 23 directly below the focusing means 422, wherein thelateral center position of the laser processed groove 24 formed alongthe predetermined division line 23 is set as a laser beam applyingposition where the laser beam is applied from the focusing means 422. Atthis time, one end (left end as viewed in FIG. 7A) of the predetermineddivision line 23 is positioned directly below the focusing means 422.Further, a focal point P of the pulsed laser beam to be applied from thefocusing means 422 is set inside the substrate 20 at an intermediateposition in the direction along the thickness of the substrate 20.Thereafter, the pulsed laser beam is applied from the focusing means 422to the wafer 2, and the chuck table 41 is moved in the direction shownby an arrow X1 in FIG. 7A at a predetermined feed speed. In thismodified layer forming step, the wavelength of the pulsed laser beam isset to a transmission wavelength to the substrate 20. When the other end(right end as viewed in FIG. 7B) of the predetermined division line 23reaches the position directly below the focusing means 422 as shown inFIG. 7B, the application of the pulsed laser beam is stopped and themovement of the chuck table 41 is also stopped.

By performing the modified layer forming step, a modified layer 26 isformed in the substrate 20 along the laser processed groove 24 formedalong the predetermined division line 23 as shown in FIG. 7C. Themodified layer forming step mentioned above is similarly performed alongall of the division lines 23 of the semiconductor wafer 2 processed bythe functional layer removing step.

For example, the modified layer forming step mentioned above isperformed under the following processing conditions.

Light source: YAG laser

Wavelength: 1064 nm

Repetition frequency: 100 kHz

Average power: 0.3 W

Focused spot diameter: φ1 μm

Work feed speed: 200 mm/second

After performing the modified layer forming step as mentioned above, anexternal force is applied to the semiconductor wafer 2 to thereby dividethe semiconductor wafer 2 along each division line 23 where the modifiedlayer 26 is formed. That is, since the modified layer 26 as a divisionstart point is formed along each division line 23 on the semiconductorwafer 2, the semiconductor wafer 2 can be easily divided along eachdivision line 23 by applying the external force. As a modification, themodified layer 26 may be formed over the entire thickness of thesubstrate 20 of the semiconductor wafer 2 along each division line 23 inthe modified layer forming step, thereby almost dividing the substrate20 along each division line 23.

A third preferred embodiment of the wafer dividing step will now bedescribed with reference to FIG. 8 and FIGS. 9A to 9D. The thirdpreferred embodiment of the wafer dividing step is the step of aligninga cutting blade with each division line 23 where the functional layer 21has been removed, thereby cutting the substrate 20 along each divisionline 23 (cut groove forming step). This cut groove forming step isperformed by using a cutting apparatus 5 shown in FIG. 8. As shown inFIG. 8, the cutting apparatus 5 includes a chuck table 51 for holding aworkpiece, cutting means 52 for cutting the workpiece held on the chucktable 51, and imaging means 53 for imaging the workpiece held on thechuck table 51. The chuck table 51 has an upper surface as a holdingsurface for holding the workpiece thereon under suction. The chuck table51 is movable both in the feeding direction shown by an arrow X in FIG.8 by feeding means (not shown) and in the indexing direction shown by anarrow Y in FIG. 8 by indexing means (not shown).

The cutting means 52 includes a spindle housing 521 extending in asubstantially horizontal direction, a rotating spindle 522 rotatablysupported to the spindle housing 521, and a cutting blade 523 mounted onthe rotating spindle 522 at a front end portion thereof. The rotatingspindle 522 is adapted to be rotated in the direction shown by an arrow523 a by a servo motor (not shown) provided in the spindle housing 521.The cutting blade 523 is composed of a disk-shaped base 524 formed ofaluminum and an annular cutting edge 525 mounted on one side surface ofthe base 524 along the outer circumference thereof. The annular cuttingedge 525 is an electroformed diamond blade produced by bonding diamondabrasive grains having a grain size of 3 to 4 μm with nickel plating tothe side surface of the outer circumferential portion of the base 524.For example, the cutting edge 525 has a thickness of 30 μm and an outerdiameter of 50 mm.

The imaging means 53 is mounted on a front end portion of the spindlehousing 521 and includes illuminating means for illuminating theworkpiece, an optical system for capturing an area illuminated by theilluminating means, and an imaging device (CCD) for imaging the areacaptured by the optical system. An image signal output from the imagingmeans 53 is transmitted to control means (not shown).

In performing the cut groove forming step by using the cutting apparatus5 mentioned above, the semiconductor wafer 2 processed by the functionallayer removing step is placed on the chuck table 51 in the conditionwhere the dicing tape 30 is in contact with the chuck table 51 as shownin FIG. 8. Thereafter, suction means (not shown) is operated to hold thesemiconductor wafer 2 through the dicing tape 30 on the chuck table 51under suction (wafer holding step). Accordingly, the SiO₂ film 211formed on the front side of the functional layer 21 of the semiconductorwafer 2 held on the chuck table 51 is oriented upward. Although theannular frame 3 supporting the dicing tape 30 is not shown in FIG. 8,the annular frame 3 is held by suitable frame holding means provided onthe chuck table 51. Thereafter, the chuck table 51 holding thesemiconductor wafer 2 is moved to a position directly below the imagingmeans 53 by operating the feeding means (not shown).

In the condition where the chuck table 51 is positioned directly belowthe imaging means 53, an alignment step is performed by the imagingmeans 53 and the control means (not shown) to detect a subject area ofthe semiconductor wafer 2 to be cut. In this alignment step, the imagingmeans 53 images the laser processed groove 24 formed along each divisionline 23 of the semiconductor wafer 2 by the functional layer removingstep. More specifically, the imaging means 53 and the control meansperform image processing such as pattern matching for making thealignment of the cutting blade 523 and the laser processed groove 24formed along each division line 23 extending in a first direction on thefunctional layer 21 of the semiconductor wafer 2, thus performing thealignment of a cut area by the cutting blade 523 (alignment step).Similarly, the alignment of a cut area by the cutting blade 523 isperformed for the other laser processed groove 24 formed along eachdivision line 23 extending in a second direction perpendicular to thefirst direction on the functional layer 21.

After performing the alignment step mentioned above to detect the laserprocessed groove 24 formed along each division line 23 of thesemiconductor wafer 2 held on the chuck table 51, the chuck table 51 ismoved to a cut start position in the cut area by the cutting blade 523,thereby positioning one end (left end as viewed in FIG. 9A) of apredetermined one of the division lines 23 on the right side of aposition directly below the cutting blade 523 by a predetermined amount.Since the laser processed groove 24 formed along each division line 23is directly imaged by the imaging means 53 to detect the cut area in thealignment step mentioned above, the lateral center position of the laserprocessed groove 24 formed along the predetermined division line 23 canbe reliably set so as to be opposed to the outer circumference of thecutting blade 523.

In the condition where the semiconductor wafer 2 held on the chuck table51 is set at the cut start position in the cut area as described above,the cutting blade 523 is lowered from a standby position shown by aphantom line in FIG. 9A to a working position shown by a solid line inFIG. 9A as shown by an arrow Z1 in FIG. 9A. As shown in FIGS. 9A and 9C,this working position is set so that the lower end of the cutting blade523 reaches the dicing tape 30 attached to the back side of thesemiconductor wafer 2.

Thereafter, the cutting blade 523 is rotated in the direction shown byan arrow 523 a in FIG. 9A at a predetermined rotational speed, and thechuck table 51 is moved in the direction shown by an arrow X1 in FIG. 9Aat a predetermined feed speed. When the other end (right end as viewedin FIG. 9B) of the predetermined division line 23 reaches a position onthe left side of the position directly below the cutting blade 523 by apredetermined amount as shown in FIG. 9B, the movement of the chucktable 51 is stopped. As a result, a cut groove 27 is formed in thesubstrate 20 of the semiconductor wafer 2 along the laser processedgroove 24 formed along the predetermined division line 23 so that thedepth of the cut groove 27 reaches the back side of the substrate 20 ofthe semiconductor wafer 2 as shown in FIG. 9D, thus fully cutting thesubstrate 20 over the entire thickness thereof (cut groove formingstep).

Thereafter, the cutting blade 523 is raised from the working position tothe standby position as shown by an arrow Z2 in FIG. 9B, and the chucktable 51 is next moved in the direction shown by an arrow X2 in FIG. 9Bto the position shown in FIG. 9A. Thereafter, the chuck table 51 ismoved in the direction (indexing direction) perpendicular to the sheetplane of FIG. 9A by an amount corresponding to the pitch of the divisionlines 23, thereby aligning the cutting blade 523 with the next divisionline 23 to be cut. In the condition where the cutting blade 523 isaligned with the next division line 23 to be cut as mentioned above, thecut groove forming step is performed similarly.

For example, the cut groove forming step mentioned above is performedunder the following processing conditions.

Cutting blade: outer diameter: 50 mm, thickness: 30 μm

Rotational speed of the cutting blade: 20000 rpm

Cut feed speed: 50 mm/second

The cut groove forming step mentioned above is performed similarly alongall of the division lines 23 of the semiconductor wafer 2. As a result,the semiconductor wafer 2 is cut along all of the division lines 23 andthereby divided into the individual devices 22.

The present invention is not limited to the details of the abovedescribed preferred embodiments. The scope of the invention is definedby the appended claims and all changes and modifications as fall withinthe equivalence of the scope of the claims are therefore to be embracedby the invention.

What is claimed is:
 1. A wafer processing method for dividing a waferinto a plurality of individual devices along a plurality of crossingdivision lines formed on a front side of said wafer, said waferincluding a substrate, a functional layer formed on the front side ofsaid substrate, and an SiO₂ film formed on a front side of saidfunctional layer, said individual devices being formed from saidfunctional layer and partitioned by said division lines, said waferprocessing method comprising: a functional layer removing step ofapplying a laser beam to said wafer along each division line to therebyremove said functional layer along each division line, said laser beamhaving an absorption wavelength to said SiO₂ film with high absorptivitydue to the stretching vibration of an O—H bond or a C—H bond remainingin said SiO₂ film; and a wafer dividing step of processing saidsubstrate along each division line where said functional layer has beenremoved by performing said functional layer removing step, therebydividing said wafer into said individual devices.
 2. The waferprocessing method according to claim 1, wherein the wavelength of saidlaser beam to be applied in said functional layer removing step is setto 2.6 to 3.5 μm.
 3. The wafer processing method according to claim 1,wherein said wafer dividing step comprises the step of applying a laserbeam having an absorption wavelength to said substrate along eachdivision line where said functional layer has been removed, therebyablating said substrate to form a division groove on said substratealong each division line.
 4. The wafer processing method according toclaim 1, wherein said wafer dividing step comprises the step of applyinga laser beam having a transmission wavelength to said substrate alongeach division line where said functional layer has been removed in thecondition where a focal point of said laser beam is set inside saidsubstrate, thereby forming a modified layer inside said substrate alongeach division line.
 5. The wafer processing method according to claim 1,wherein said wafer dividing step comprises the step of aligning acutting blade with each division line where said functional layer hasbeen removed, thereby cutting said substrate along each division line.